Tom - perhaps sometime you'll realise that train formations can change. HSTs used to be 7/8 coaches and are now 8/9, having been down to 3 on various occasions; Virgin have had extra Pendolino coaches (from 8 to 9 coaches), and seem likely to get more (11); Sprinters and their followers have been reformed on numerous occasions (2 to 3 mostly). It's not impossible to imagine 5 coach 450s/455s or even 4+4+2 sets.
As to the original question, SWT tell us
here that there are "plans to introduce 10-car trains on Windsor to Waterloo services in 2010". So not for a while.